1. Field of the Invention
The present invention relates to a polymer having the structure of formula (I) as given herein, and a negative-acting photoimageable composition comprising the polymer, which is capable of being developed in an aqueous alkaline solution. The photoimageable composition of the present invention can be used as a photoresist in the processes of acid electroplating, covering pores, acid and alkaline etching, gold plating, electroless nickel immersion gold (ENIG) for the production of printed circuit boards.
2. Description of the Prior Art
In photoimageable compositions applied to printed circuit boards, different constituents result in different applications of the photoimageable compositions. Better flexibility and toughness may enhance the tensile properties and ductility of the photoimageable compositions. Generally, printed circuit boards have orifices, enabling the circuit on one side of the boards to be connected to the circuit on the other side. The photoimageable composition layer must cover these orifices during the process, and since the orifices in the printed circuit boards are becoming bigger and bigger gradually, the covering strength is more and more important. In the past, the photoimageable compositions were poor in the capacities of electroplating resistance, tensile, alkaline etching resistance, gold plating resistance, and nickel plating resistance, thus the appliance thereof in the printed circuit boards was susceptible to be limited. In the specification of U.S. Pat. No. 4,555,552, it is mentioned that after being mixed with elastic resins, the polyurethane polymerized from an alcohol compound having an aromatic ring and a diisocyanate may enhance the tensile strength and ductility. It has been found after extensive studies that the polyurethane that has a bisphenol group is one of the important components which can improve the flexibility and toughness of the photoimageable composition, and will improve not only the tensile properties, but also the capacities of electroplating resistance, alkaline etching resistance, gold plating resistance, and nickel plating resistance, and thus increases the processability of the printed circuit boards.
Furthermore, in the specification of U.S. Pat. No. 6,322,951, it is mentioned that it has been found that the flexibility of a photoimageable composition of a printed circuit board is enhanced and even the ability of stripping is correspondingly improved, if the photoimageable composition contains a polyurethane (meth)acrylate having at least two alkoxy functional groups and one ring-opened lactone functional group.
The inventors utilize different properties of different functional groups to enhance different properties of photoimageable compositions by combining different functional groups, for example, adding an alkoxy functional group and an ring-opened lactone functional group to a polyurethane (meth)acrylate to increase the chain length, and thus to enhance the flexibility of the photoimageable composition; and, adding an aromatic ring functional group to the polyurethane (meth)acrylate to enhance the tensile strength and ductility of the photoimageable composition due to the rigidity of the aromatic ring. The polyurethane (meth)acrylate synthesized at the most preferred balance point of different properties can be used in photoimageable compositions for enhancing the capacities of electroplating resistance, covering pores, nickel/gold plating resistance, acid and alkaline etching resistance, and electroless nickel plating immersion gold resistance.